Variable Optical Attenuators (VOAs) are used in fiber optic systems to control the amount of light falling, for example, on a detector. For this purpose, it is known to use an asymmetric style MEMS electro thermal actuator. This can be fabricated using bulk silicon-on-insulator wafers. A known asymmetric actuator consists of a cantilevered electrical loop having a narrow hot arm, a wide cold arm and a narrow flexure element. When current is passed through the loop, the narrow arm heats up more than the wide cold arm, and as a result causes the actuator to flex about the narrow flexure portion connecting the movable arms to the stationary substrate. A typical prior art MEMS thermal actuator is described in U.S. Pat. No. 6,275,320, dated Aug. 14, 2001, the contents of which are herein described by reference.
In order to obtain as large a deflection as possible, the variables that affect both the heat transfer and deflection mechanics should be optimized. Intuitively, it may seem desirable to make the wide arm as narrow as possible to facilitate more bending while keeping it wide enough to maintain the low temperature. However, according R. Hickey et. al., the cold arm should be as stiff as possible in order to force all the rotation to take place near in the flexure portion, thereby increasing the deflection.
It is known that a fully suspended actuator can be realized by using silicon on insulator wafers and fully etching the backside handle wafer under the device. The use of a fully suspended structure eliminates the possibility of stiction to the substrate, ensures that the buried oxide can be fully etched and reduces the power required to move the actuator by eliminating the large heat load created by a thick redundant substrate. Fully suspending the actuator in this manner removes the underlying substrate heat sink. However, the problem with removing the substrate heat sink is that the thermal gradients set up within the air surrounding the device become altered. A large envelope of quiescent air surrounds the hot arm. Without the substrate, a significant amount of heat is transferred through the air to the cold arm. As a result, the cold arm expands, the moment decreases and the actuator deflection drops.
It would in theory be possible to separate the two arms separated further. However, separation of the two arms reduces the amount of deflection.